I have both the "Download Instant Messenger" and just "Instant Messenger" icon. I'm assuming, as of right now, both of the icons lead me to the same place, since they both open up a browser. Now, from what I'm guessing is that this will get me OZ Instant Messenger but, when it opens the browser it says:
"This application is only for the T-Mobile BlackBerry Pearl 8100. The device you are using is not supported." I don't get why it says this, because that's what I have.

I have both the "Download Instant Messenger" and just "Instant Messenger" icon. I'm assuming, as of right now, both of the icons lead me to the same place, since they both open up a browser. Now, from what I'm guessing is that this will get me OZ Instant Messenger but, when it opens the browser it says:
"This application is only for the T-Mobile BlackBerry Pearl 8100. The device you are using is not supported." I don't get why it says this, because that's what I have.

After upgrading to 4.5 on my 8100, I got the same thing. I just re-installed OZ and set up my firewall to accept the connection, now I have AIM working again.

Alright, so you don't have to go what I've gone through, i'll summarize this whole deal.

OZ Instant Messenger is a messenger for your phone that can connect to AIM, MSN, Yahoo, and ICQ. It uses your text messages instead of a data plan (which is why I was looking for it, since I can't afford the data plans available.)

You can download OZ Instant Messenger by going to http://nig.gr/144t on your phone, and it'll download automatically. You can probably get AIM OTA also, but I had it in my desktop manager after upgrading to 4.5OS. I'm assuming that AIM would be faster than OZ since it uses data, but if you don't have a data plan then don't bother.